In a digitally controlled printing system, a print medium is directed through a series of components. The print medium can be in the form of cut sheets or a continuous web. As the print medium moves through the printing system, colorant, for example, ink, is applied to the print medium by one or more printing stations. In the case of an inkjet printer, the colorant is a liquid ink, and the printing process is commonly referred to as jetting of the ink.
In commercial inkjet printing systems, the print medium is physically transported through the printing system at a high rate of speed. For example, the print medium can travel 650 to 1000 feet per minute. Inkjet lineheads in commercial inkjet printing systems typically include multiple printheads that jet ink onto the print medium as the print medium is being physically moved through the printing system. A reservoir containing ink or some other material is usually behind each nozzle plate in a linehead. The ink streams through the nozzles in the nozzle plates when the reservoirs are pressurized.
The printheads in each linehead in commercial printing systems typically jet only one color. Thus, there is a linehead for each colored ink when different colored inks are used to print content. For example, there are four lineheads in printing systems using cyan, magenta, yellow, and black colored inks. The content is printed by jetting the colored inks sequentially, and each colored ink deposited on the print medium is known as a color plane. The color planes need to be aligned (i.e., “registered” with each other) so that the overlapping ink colors produce a quality single image.
Color registration errors can be classified into different types. Examples of color registration errors include, but are not limited to, a color plane having a linear translation with respect to another color plane, a color plane being rotated with respect to another color plane, and a color plane being stretched, contracted, or both stretched and contracted in different regions or in different directions with respect to another color plane.
There are several variables that contribute to the registration errors in color plane alignment including physical properties of the print medium, conveyance of print medium, ink application system, ink coverage, and drying of ink. Color registration errors are typically managed by controlling these variables. However, controlling these variables can often restrict the range of desired print applications. For example, color-to-color registration errors will typically become larger as paper weight for the print application is reduced, when ink coverage is increased, or when the amount of ink coverage is more variable for successive documents. These limitations compromise the range of suitable applications for inkjet printing systems.
As discussed in U.S. Pat. No. 9,094,643 (Armbruster et al.), entitled “Color-to-color correction in a printing system,” when a print job consists of printing multiple copies of a repeated sequence of pages, such as multiple copies of a multi-page document such as book, the registration errors are quite consistent from one cycle of the sequence of pages to the next cycle. Recognizing the consistency of the registration errors, Armbruster et al. disclosed a method for improving the color plane registration by compensating for such cyclically repeating registration errors. The method included measuring the registration errors for each page in a first copy of the sequence of pages, and determining registration correction values associated with each of the pages. While printing subsequent copies of the multi-page document, the registration of each page is corrected using the determined registration correction values associated with that page. This method was found to be quite effective at reducing the registration errors. However, it requires that the print engine know the number of pages in a document before it starts printing. In some job submission methods, such as Intelligent Printer Data Stream (IPDS) or Inkjet Printing Data Stream (UPDS), the number of pages in the document is not provided in the data stream. There remains a need for improved registration correction methods to correct print jobs including a plurality of multi-page documents where the number of pages in each document is not known.